FIG. 7 is a configuration explanatory view showing an example of a field wireless system which has been conventionally used. In FIG. 7, an antenna 1 is installed in a field wireless device, not shown, serving as a transmitting station and an antenna 2 is installed in a field wireless device, not shown, serving as a receiving station. The antennas 1 and 2 are installed to face each other across a tank 3. The field wireless devices, not shown, are driven by a battery and are configured to send and receive various data to and from each other by an line-of-sight low power wireless communication through the antennas 1 and 2.
However, because the tank 3 becomes an obstacle to the line-of-sight wireless communication in such a state, it is difficult to perform the line-of-sight low power wireless communication between the transmitting station and the receiving station.
Thus, antennas 4 and 5 having directionality are installed in the vicinity of the top portion of the tank 3, at which the antennas 1 and 2 can be looked down, and also a field wireless device 6 driven by a battery is installed to be connected with the antennas 4 and 5 and thus to serve as a relay station. Accordingly, the relay station performs an line-of-sight wireless communication with the transmitting station through the antennas 1 and 4 and also performs an line-of-sight wireless communication with the receiving station through the antennas 2 and 5.
Such a field wireless device 6 serving as the relay station is equipped with a communication relay unit 61, which has a receiving and transmitting function of amplifying a received signal again and then transmitting the signal after adding its own communication data thereto as needed and a routing function of designating a communication path, and a driving battery 62. Hereinafter, this is referred to as a router 6.
However, the router 6 needs to receive and retransmit communication data of other field wireless devices serving as the transmitting station and the receiving station, in addition to its own communication data, and accordingly consumes a large amount of electric power for wirelessly receiving and transmitting and for data processing.
Thus, the battery life of the router 6 is generally shortened as compared to the other field wireless devices and is likely to be about a fraction of the other field wireless device depending on conditions. If the battery life of the router 6 runs out to stop operation of the router 6, receiving and retransmitting of communication data from and to the other field wireless devices as described above are impossible so that the communication path of the field wireless system cannot be constructed.
As a result, an operable period of the field wireless system is determined by the battery life of the router 6, and, as described above, the period is shorter than battery lives of the other wireless devices and in some cases, is limited to a fraction of the battery lives.
Even though the battery lives of the other field wireless devices are remained, a battery replacement work of the router 6 is required. In this case, the function of the router 6 has to be temporarily paused due to the battery replacement, thereby causing an influence on the communication function of the system.
As a measure for avoiding this problem, a measure in which a large capacity battery is mounted in only the router 6 is conceived, but there is a problem that in addition to increase in size or weight of the battery, costs thereof is increased.
Also, because the field wireless device as the router 6 becomes different from field wireless devices, which do not have the routing function, in terms of configurations, a field wireless device placed in a field cannot be flexibly used in such a manner that depending on situations, a function setting thereof is changed over into a router or a field wireless device, which does not have the routing function.
FIG. 8 is a block diagram showing an example of a passive repeater used as a relay device which does not requires a power supply, wherein the same parts as those in FIG. 7 are designated by the same reference numerals.
As shown in FIG. 8, the passive repeater is configured to connect antennas 4 and 5 via a high frequency cable. 7. For example, radio waves absorbed by one antenna 4 are guided to the other antenna 5 via the high frequency cable 7, where the radio waves are reradiated. This action is valid in both directions.
If the passive repeater is installed for example in the vicinity of the top portion of a tank 3, an influence of the tank 3 on wireless communication between a transmitting station, which is connected with the antenna 1, and a receiving station, which is connected with the antenna 2, arranged to face each other across the tank 3 is reduced because radio waves transmitted from the antenna 1 of one station (e.g., transmitting station) are absorbed by one antenna (e.g., 4), reradiated from the other antenna (e.g., 5) and then received by the antenna 2 of the other station (e.g., receiving station) so that the radio waves are substantially relayed.
In Patent Document 1, a passive antenna type communication system technique is disclosed in which even at locations where data transmission by wireless communication is impossible, the communication is enabled without modifying wireless devices, thereby allowing field data to be monitored and controlled by a center apparatus, a mobile terminal and the like.
Also, in Patent Document 2, a technique is disclosed which can easily convert a previously installed field device into a wireless type.